This section will proceed with a suggested course of action when utilizing the
EventScheduler for dealing with deadlines.
To help understand this better lets take the scenario of a saga: It is easy to make a saga take action when something happens. After all, there is an event to notify the saga. But what if you want your saga to do something when nothing happens? That's what deadlines are used for. For invoices, that is typically several weeks, whereas the confirmation of a credit card payment should occur within a few seconds.
In Axon, you can use an
EventScheduler to schedule an event for publication. In the example of an invoice, you would expect the invoice to be paid within thirty days. A saga would, after sending the
CreateInvoiceCommand, schedule an
InvoicePaymentDeadlineExpiredEvent to be published in 30 days. The
EventScheduler returns a
ScheduleToken after scheduling an event. This token can be used to cancel the schedule, for example when a payment of an Invoice has been received.
Axon provides three
The pure-Java implementation of the
EventScheduler uses a
ScheduledExecutorService to schedule event publication. Although the timing of this scheduler is very reliable, it is a pure in-memory implementation. Once the JVM is shut down, all schedules are lost. This makes this implementation unsuitable for long-term schedules. The
SimpleEventScheduler needs to be configured with an
EventBus and a
SchedulingExecutorService (see the static methods on the
java.util.concurrent.Executors class for helper methods).
QuartzEventScheduler is a more reliable and enterprise-worthy implementation. Using Quartz as underlying scheduling mechanism, it provides more powerful features, such as persistence, clustering and misfire management. This means event publication is guaranteed. It might be a little late, but it will be published. It needs to be configured with a Quartz
Scheduler and an
EventBus. Optionally, you may set the name of the group that Quartz jobs are scheduled in, which defaults to
AxonServerEventScheduler uses Axon Server to schedule events for publication. As such, it is a hard requirement to use Axon Server as your Event Store solution to utilize this event scheduler. Just as the
AxonServerEventScheduler is a reliable and enterprise-worthy implementation of the
EventScheduler interface. Creating a
AxonServerEventScheduler can be done through its builder, whose sole requirement is the
It is important to note that both the
AxonServerEventScheduler should use the event
Serializer to serialize and deserialize the scheduled event. If the
Serializer used by the scheduler does not align with the
Seralizer used by the event store, exceptional scenarios should be expected. The Quartz implementation's
Serializer can be set by defining a different
EventJobDataBinder, whereas the Axon Server implementation allows defining the used
Serializer directly through the builder.
One or more components will be listening for scheduled Events. These components might rely on a transaction bound to the thread that invokes them. Scheduled events are published by threads managed by the
EventScheduler. To manage transactions on these threads, you can configure a
TransactionManager or a
UnitOfWorkFactory that creates a transaction bound unit of work.
Spring users can use the
SimpleEventSchedulerFactoryBeanfor easier configuration. It allows you to set the
Spring Boot users which rely on Axon Server do not have to define anything. The auto configuration will automatically create a